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“The rapidly developing science and technology of graphene and atomically-thin materials has taken another step forward with new research from The University of Manchester. This research, published in Science, shows how a variety of different electronic properties – essentially new materials - can be realised simply by applying a magnetic field. Electrons inside materials move quite differently from a free electron in vacuum: their properties are strongly affected by the electric potential of ions comprising the crystal lattice. This interaction changes the mass of electrons and makes materials either metals, semiconductors or insulators, depending on the detailed atomic structure. This provides the great variety of material properties we know and work with. Earlier, the researchers at The University of Manchester have found ways to create new materials with bespoke electronic properties by placing one electronic material (in this case graphene) on top of another crystal, hexagonal boron nitride. Now, they demonstrate how to create a whole sequence of different electronic materials by simply tuning the applied magnetic field. In this combination of materials, boron nitride atoms create a periodic pattern for electrons in graphene known as a superlattice. Such a superlattice is characterised by the length scale of the periodic pattern, whereas the strength of applied magnetic field can be counted in so-called flux quanta, elementary units of magnetic field. A matching condition is achieved each time when an integer fraction of the flux quantum penetrates through an area given by the elementary superlattice. At these special values of magnetic field, the researchers observed that electrons started moving along straight lines, as if the magnetic field were absent. This is in stark contrast to the known behaviour of electrons in a magnetic field where electrons must move along curved trajectories known as cyclotron orbits. As a result of these changes from straight to curved trajectories and back at many matching conditions, the researchers found oscillations in electrical conductivity of graphene superlattices.”

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